High gradient magnetic separators are used to separate weakly magnetic materials of very small particle size from a liquid or gaseous suspension medium, or from a mixture of non-magnetic particles carried by a liquid or gaseous medium. High gradient separators are usually based on the use of a matrix of finely divided filamentary ferromagnetic material such as steel wool, woven or felted steel fibers, expanded steel lath, or the like. Such matrices typically have filament sizes comparable to the size of particles being separated, and typically comprise 95% void space even when highly compressed, so that they offer a very considerable surface area and open flow volume. Such matrices cannot be effectively magnetized in the magnetic circuit of conventional separation magnets because the matrices do not conduct magnetic flux and require very strong background fields to magnetize; thus high-performance, iron-bound solenoid magnets are used. These iron-bound solenoid magnets generate strong uniform background fields in a cylindrical volume surrounded by coils carrying high-density electric current: the cylindrical volume is terminated at each end by a massive iron pole plate connected to an external flux return frame which surrounds the coil structure.
In cyclic high gradient magnetic separators, the matrix material is packed into a cylindrical canister which fits into the magnet structure. The material to be separated is fed through this canister by means of one or more inlet and outlet ducts which pass through the massive pole plates. Magnetic particles are retained at the surface of the magnetized matrix filaments if the magnetic trapping forces are greater than the viscous drag forces at the flow velocity. When the matrix is loaded, typically when it has collected about its own weight in magnetic particles, the magnetic field is turned off and the collected magnetic particles are flushed out of the canister, preferably at a flow rate several times higher than the feed rate. The collecting cycle is then repeated.
In continuous high gradient magnetic separators, the matrix material is carried continuously through the magnet structure, for example by means of a compartmented ring or carousel. For this purpose, the iron-bound solenoid has the shape of a circular segment rather than a cylinder. The feed is introduced through ducts in the pole structure as in the cyclic devices, but the magnetic materials are flushed out of the matrix continuously at a point outside the magnet structure. Continuous devices are needed for applications where the magnetics comprise a sizable fraction of the feed, such as in the beneficiation of weakly magnetic, finely divided iron ore. Cyclic devices are more expedient where only a small fraction of the feed is magnetic, such as the purification of clay used in paper coating, or the purification of water.
The feed ducts which pass through the magnet pole plates necessarily have a substantially smaller flow cross section than the matrix canister, the ratio of cross section areas typically ranging from 8 to 24, depending on the particular application in question. Typical flow velocities in the canister range from 1 to 30 cm/sec (2 to 60 ft./min.) Flow velocity is an important process parameter since the viscous drag forces which compete with the magnetic trapping forces on particles increase approximately as the square of flow velocity, and since the maximum permissible flow velocity determines the capacity and thus the operating cost of a machine.
Therefore a uniform flow distribution should be maintained throughout the canister volume. Mal-distribution will not only reduce the performance of a machine, it will also initiate a progressively deteriorating instability. Slurry particles will settle and accumulate in low velocity regions and cause local clogging of the matrix, thereby forcing the flow to concentrate progressively into tunnels of high velocity. In some applications this inherent instability is so severe that local clogging will develop in a matrix even if flow is distributed uniformly at the inlet; it is necessary in such cases to re-distribute the flow at several points along its path through the matrix.
High gradient magnetic separators therefore require flow distributors which are very effective, and which satisfy several other requirements not encountered in other applications. Such flow distributors should occupy a minimum of space in the flow direction, because at least one or even several of them must be located inside the very expensive magnetized volume. They must permit flushing velocities several times higher than the feed velocity without causing a prohibitive pressure drop. They must be readily adaptable to a wide range of flow conditions, slurry viscosities, etc. Finally, they must operate without causing stagnation and clogging.
A conventional approach provides a plenum chamber at the canister inlet which receives the slurry input and terminates in a perforated plate or screen covering the entire inlet cross section of the matrix canister. This perforated plate or screen must offer enough flow resistance to dissipate substantially all the kinetic energy or "velocity head" of the incoming fluid and achieve the desired degree of uniformity.
This method of flow distribution by a perforated plate or screen is unsatisfactory for several reasons. Even if the high pressure drop is acceptable during the feed cycle, as is the case in applications involving moderate feed rates, this pressure drop makes it impossible to achieve sufficiently high flushing velocities to effectively purge the matrix. Another serious problem is that the sudden decrease in flow velocity as the slurry enters the plenum induces sedimentation and cumulative clogging.